Explosive caldera eruptions Explosive caldera eruptions are produced by a magma chamber whose
magma is rich in
silica. Silica-rich magma has a high
viscosity, and therefore does not flow easily like
basalt. When the magma approaches the surface of the Earth, the drop in
confining pressure causes the trapped gases to rapidly bubble out of the magma, fragmenting the magma to produce a mixture of
volcanic ash and other
tephra with the very hot gases. The mixture of ash and volcanic gases initially rises into the atmosphere as an
eruption column. However, as the volume of erupted material increases, the eruption column is unable to
entrain enough air to remain buoyant, and the eruption column collapses into a tephra fountain that falls back to the surface to form
pyroclastic flows. Eruptions of this type can spread ash over vast areas, so that ash flow
tuffs emplaced by silicic caldera eruptions are the only volcanic product with volumes rivaling those of
flood basalts. Eruptions forming even larger calderas are known, such as the
La Garita Caldera in the
San Juan Mountains of
Colorado, where the
Fish Canyon Tuff was blasted out in eruptions about 27.8 million years ago. The caldera produced by such eruptions is typically filled in with tuff,
rhyolite, and other
igneous rocks. The caldera is surrounded by an
outflow sheet of ash flow tuff (also called an
ash flow sheet). If magma continues to be injected into the collapsed magma chamber, the center of the caldera may be uplifted in the form of a
resurgent dome such as is seen at the
Valles Caldera,
Lake Toba, the San Juan volcanic field,
Cerro Galán,
Yellowstone, and many other calderas. or
Mount Pinatubo in 1991, may result in significant local destruction and a noticeable
drop in temperature around the world. Large calderas may have even greater effects. The ecological effects of the eruption of a large caldera can be seen in the record of the
Lake Toba eruption in
Indonesia. At some points in
geological time, rhyolitic calderas have appeared in distinct clusters. The remnants of such clusters may be found in places such as the
Eocene Rum Complex of Scotland,
Valles For their 1968 paper and it remains one of the best studied examples of a resurgent caldera.
Toba About 74,000 years ago, this Indonesian volcano released about
dense-rock equivalent of ejecta. This was the largest known eruption during the ongoing
Quaternary period (the last 2.6 million years) and the largest known explosive eruption during the last 25 million years. In the late 1990s,
anthropologist Stanley Ambrose proposed that a
volcanic winter induced by this eruption reduced the human population to about 2,000–20,000 individuals, resulting in a
population bottleneck. More recently,
Lynn Jorde and
Henry Harpending proposed that the human species was reduced to approximately 5,000–10,000 people. There is no direct evidence, however, that either theory is correct, and there is no evidence for any other animal decline or extinction, even in environmentally sensitive species. There is evidence that human habitation continued in
India after the eruption. in the
Galápagos archipelago , Van Lake, Eastern Turkey
Non-explosive calderas Caldera, located in central Chile near the border with Argentina, filled with ice. The volcano is in the southern Andes Mountains within Chile's Parque Nacional Villarica. Some volcanoes, such as the large
shield volcanoes
Kīlauea and
Mauna Loa on the island of
Hawaii, form calderas in a different fashion. The magma feeding these volcanoes is
basalt, which is silica poor. As a result, the magma is much less
viscous than the magma of a rhyolitic volcano, and the magma chamber is drained by large lava flows rather than by explosive events. The resulting calderas are also known as subsidence calderas and can form more gradually than explosive calderas. For instance, the caldera atop
Fernandina Island collapsed in 1968 when parts of the caldera floor dropped . ==Extraterrestrial calderas==